Particles with diameters of less than 100 nm (hereinafter referred to as nanoparticles) are known to exhibit properties different from those of particles with diameters of 1 μm or more. In recent years, such properties (for example, low-temperature sinterability of particles) are being utilized in the field of electric and electronic devices to reduce their size.
The methods for manufacturing nanoparticles are broadly classified into two types, i.e., the liquid phase method in which nanoparticles are produced in a liquid and the gas phase method in which nanoparticles are formed in a gas phase. The production in a gas phase is generally performed in a high-vacuum state, and therefore a large-scale apparatus must be used. Accordingly, the gas phase method has problems in that the initial investment is high and that the method is not suitable for mass production. The problems in the gas phase method are less likely to occur in the liquid phase method, but the particles produced by the liquid phase method may have large variation and are not preferably used in some applications.
Particularly, when nanoparticles are produced by a conventional method in which a reducing agent is added at once to a solution of a metal salt, the nucleation and growth of the metal particles cannot be easily controlled, and therefore it is difficult to obtain particles with a uniform diameter.
In addition, to avoid spontaneous sintering of nanoparticles, the particles must be subjected to anti-sintering treatment by forming a coating component on their surfaces. The anti-sintering treatment must be performed at the initial stage of the reaction to maintain the independence of the particles. However, the use of the surface coating component without any treatment in the environment in which the production reaction proceeds at once as described above causes greater difficulty in obtaining uniformity of the particles.
Various methods have been used to solve the above problems. More specifically, one exemplary method (see, for example, Patent Document 1) uses slow reduction, and another exemplary production method (see, for example, Patent Document 2) uses a continuous flow scheme. Moreover, in the technology (see Patent Document 3), to obtain silver flakes with a uniform particle diameter, although the diameter does not fall within the particle diameter range, a foreign component (copper) is used to increase the particle size and the crystallinity.